The present invention relates generally to atomic absorption spectroscopy and more particularly to a furnace for electrothermal atomization of samples in atomic absorption spectroscopy.
Electrothermal atomizers, commonly referred to as heated graphite atomizers or graphite furnaces, are utilized in atomic absorption spectrophotometers for rendering the sample to be analyzed in atomic form. Typically, the furnaces comprise a tubular graphite member clamped between annular graphite contacts or electrodes engaging its respective ends. A radial aperture in the side wall of the tubular member at the mid-point of its length serves as a sample port accommodating the insertion of the substance to be analyzed into the tubular member.
The contacts, usually mounted in cooling jackets, are pressed into tight engagement with the ends of the tubular furnace member by resilient biasing means or a servomotor. An intense electrical current, passed longitudinally through the tubular member between the contacts heats the member to the high temperature required to convert the sample to a "cloud of atoms".
A measuring light beam of a line emitting light source which comprises the resonant spectral line of a looked-for element is passed through the annular graphite contacts and the longitudinal bore of the graphite tube. The amount of the looked-for element in the sample can be determined from the absorption of the measuring light beam.
In order to prevent rapid deterioration of the tubular graphite member by oxidation at the high temperatures required for atomization of the analyte, provision is made for enveloping it in a flow of inert protective gas. The graphite tube is surrounded by an inert gas such that oxygen does not get into contact with the graphite tube.
A non-uniform temperature distribution along the graphite tube results when the graphite tube is held at its ends. The graphite tube has a higher temperature in its central area than at the ends where the heat dissipates to the cooled contacts. This non-uniformity of temperature results in the deposition of sample on the cooler ends of the tubular member; the deposit is re-evaporated in subsequent use of the tubular member thereby contaminating the new sample.
A graphite furnace of the type just described is shown in Braun et al., U.S. Pat. No. 4,022,530 which is incorporated herein by reference. In this particular furnace, the contacts are tubular rather than annular. The two contacts extend around the graphite tube along its entire length between the contact surfaces except for a separating gap. An inert gas flow is passed into the graphite tube from both ends. This inert gas flow emerges through a radial bore of the graphite tube in its center. One of the tubular contacts has a radial bore which is aligned with the radial bore of the graphite tube.
In an attempt to achieve a more advantageous temperature distribution along the graphite tube, it has been proposed to pass the heating current transversely through the graphite tube rather than longitudinally. For this purpose, a contact arrangement is described in Woodriff, U.S. Pat. No. 4,407,582 wherein two pairs of interconnected contacts in the form of fork-shaped contact pieces are employed which engage the graphite tube radially on opposite sides. The heating current flows in a circumferential direction through the graphite tube in the area of the ends. The graphite tube is heated in the area of its ends and heat flows from the ends to the center to obtain a more uniform temperature distribution.
In this known contact arrangement, the electrodes engage the hot parts of the graphite tube; consequently, the reproducibility of the contact characteristics is poor. Furthermore, it is difficult to protect the graphite tube from exposure to atmospheric oxygen by means of an inert protective gas flow, resulting in short useful life of the graphite tube.
In Hutsch et al., U.S. Pat. No. 4,726,678 which is incorporated herein by reference and the publication in "Analytical Chemistry" 58 (1986), 1973, a graphite furnace is described in which the tubular furnace body has a rectangular cross-section and contact projections extend transversely to the axis of the furnace body. The furnace body and contact projections are formed as one integral graphite element. The contacting is provided in a cold area at planar contact surfaces.
In some embodiments of U.S. Pat. No. 4,726,678, the contact projections between the contact surfaces and furnace body have areas of a reduced cross-section. In one embodiment (FIG. 3), cutouts are provided which extend longitudinally to the furnace axis. These cutouts provide a reduction of the heat dissipation from the furnace body to the ends of the contact pieces and match electrical resistance to the output of the electrical power supply. In another embodiment (FIG. 4), it is stated that the contact projections are provided with multiple apertures and that this configuration is specifically suited for setting predetermined temperature profiles. Current is supplied at certain locations and flows through the furnace body in order to generate Joul's heat. This is similar to the arrangement of U.S. Pat. No. 4,407,582, except that the contacting is displaced to a cooler area.
In the known arrangement, the power is supplied along the tubular furnace body non-uniformly.
In German Pat. Application No. P 37 35 013.7, not pre-published, an electrothermal furnace is disclosed which comprises a tubular furnace body with contact projections provided on opposite sides and contact surfaces. The contact projections have longitudinal contact ribs and cylindrical contact projections with conical contact surfaces integral therewith. The contact ribs have contractions and are trapezoidal in plan view on the surface of the contact rib. The long parallel side of the trapezoid is adjacent to the furnace body while the contact projection is adjacent to the short parallel side of the respective trapezoid. The contact ribs have longitudinal cylindrical cutouts on both sides. The axes of the cutouts extend parallel to the axis of the furnace body in order to form the contractions. These cutouts however do not extend throughout the entire length of the contact ribs, but end at distance from a transverse center plane. In this manner, reinforcing ribs are formed in the center on both sides of the contact ribs. The reinforcing ribs extend along the transverse center plane perpendicular to the axis of the furnace and are connected to the contact ribs and the furnace body.
In German Pat. Application No. P 37 35 013.7, a platform for accommodation of samples is described which can be inserted into the furnace and which is heated only indirectly by the furnace. Contacts are provided between which the furnace is held and through which current is passed transversely through the furnace body. The contacts form a cavity into which inert gas is passed.
From German Pat. Application No. P 37 43 286.9, also not pre-published (corresponding to Tamm, U.S. Ser. No. 285,884 filed Dec. 16, 1988, commonly owned and incorporated herein by reference), an electrothermal furnace is disclosed which has a tubular furnace body with a lateral inlet port and a hollow, generally semicylindrical inner body. The inner body is arranged opposite the inlet port and is connected to the furnace body by one single web.
In Tamm et al., U.S. Pat. No. 3,862,805, a longitudinally heated graphite tube is disclosed wherein the inner wall has a plurality of annular cutouts or collars. There is also described a graphite tube in which the cutouts are arranged helically and virtually form a kind of a threaded portion.
In Tamm, U.S. Pat. No. 4,111,563, a longitudinally heated graphite tube is disclosed in which a tubular inner body made of graphite is held in a tubular outer body (furnace body) by radially extending ribs on opposite sides. The inner body is substantially shorter than the outer body and is arranged in the central area of the outer body.
It is an object of the present invention to overcome or mitigate the disadvantages of the prior art noted above and to provide a new and improved furnace for electrothermal atomization.
Another object of the invention is to provide an electrothermal atomization furnace which attains uniform temperature distribution along the tubular furnace body.
A further object is to provide such a furnace and contacting arrangement which facilitates effective protection of the furnace tube against exposure to atmospheric oxygen.
A further object of the invention is to provide such a furnace which is economical to manufacture.
Other objects will be in part obvious and in part pointed out in more detail hereinafter.
Accordingly, it has been found that the foregoing and related objects are attained in an electrothermal atomization furnace having a tubular electrothermal furnace body with a central portion and integral contact projections disposed on opposite sides of the furnace body. The contact projections have contact surfaces adapted for mounting engagement between cooperating current-supplying contacts or electrodes for passing a high electrical current through the furnace body. The contact projections define a cross-section between the contact surfaces and the furnace body with the contact projections containing bore means for reducing the cross section continuously along the central portion of the furnace body for uniform temperature distribution. In one embodiment, a plurality of bores in the contact projections extend parallel to the longitudinal axis of the furnace body to form the area of reduced cross section and are configured and disposed to produce uniform temperature distribution. In a further embodiment, a plurality of grooves are provided in the outer surface of the contact projections which extend parallel to the furnace axis to form the area of reduced cross-section continuously along the furnace body and are configured and disposed to produce a uniform temperature distribution. In both embodiments, the bores and grooves are fluidly connected to an inert gas passage for enveloping the furnace body in inert gas.
It has been found particularly advantageous that the areas of reduced cross-section extend continuously along the central part of the furnace body. In this regard, the design of the furnace according to the present invention is different from that of U.S. Pat. No. 4,726,678 where no continuous reduction of the cross-section is provided but rather discrete cutouts are provided in the contact projections, and is different from German Pat. Application No. P 37 35 013 where a reinforcing rib is formed just in the center and there reduces the electrical resistance for the supplied current.